messages: custom content described in customcontent.go, message tests

customcontent.go constains a structure with all the custom content,
methods for getting display names, subjects, etc., and a list of
variables, conditionals, and placeholder values. Tests for constructX
methods included in email_test.go, and all jfa-go tests can be run with
make INTERNAL=off test.

fs.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"io/fs"
+	"os"
+)
+
+type genericFS interface {
+	fs.FS
+	fs.ReadDirFS
+	fs.ReadFileFS
+}
+
+var localFS genericFS
+var langFS genericFS
+
+type dirFS string
+
+func (dir dirFS) Open(name string) (fs.File, error) {
+	return os.Open(string(dir) + "/" + name)
+}
+
+func (dir dirFS) ReadFile(name string) ([]byte, error) {
+	return os.ReadFile(string(dir) + "/" + name)
+}
+
+func (dir dirFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	return os.ReadDir(string(dir) + "/" + name)
+}